The source voltage produced by the rotor excitation of an elevator motor depends on the speed of rotation of the rotor. When the speed of rotation of the rotor and thereby the source voltage increases, the magnitude of the needed supply voltage of the motor also increases. The magnitude of the needed supply voltage of the motor is also affected by e.g. the motor current. Since the largest possible supply voltage of the motor is determined on the basis of the maximum output voltage of the power supply apparatus, such as of the frequency converter, of the motor, the permitted speed range of the motor must be limited such that the supply voltage required by the motor does not exceed the maximum output voltage of the power supply apparatus of the motor. Particularly in lower-powered systems, a frequency converter is usually used as the power supply apparatus of the motor, the intermediate circuit voltage of which frequency converter is unregulated and is determined directly on the basis of the voltage of the electricity network supplying the frequency converter. Since the intermediate circuit voltage, on the other hand, sets the largest possible output voltage of the frequency converter, e.g. a momentary voltage reduction occurring in the electricity network supplying the frequency converter has the effect that the output voltage of the frequency converter decreases. For this reason the rated speed of the elevator and the corresponding maximum speed of rotation of the electric motor during normal operation are usually selected such that at the rated speed and with the rated load a voltage margin remains between the output voltage of the frequency converter and the supply voltage of the motor, which margin allows e.g. a 15 percent voltage reduction in the voltage of the electricity network supplying the frequency converter. A voltage reduction in the electricity network supplying the frequency converter can be caused e.g. by a voltage reduction or electricity outage occurring in the public electricity network. Also, e.g. in connection with acceleration of the elevator. the large instantaneous current taken by the electric motor might cause a momentary voltage reduction in the supplying electricity network. The voltage endurance of the solid-state switches of the power supply apparatus of the motor also limits the maximum output voltage in those power supply apparatuses in which the intermediate circuit voltage can be regulated.
Since the speed of the elevator affects e.g. the transport capacity of the elevator system as well as the door-to-door time, within the framework of which a passenger can be moved from the departure floor to the destination floor, it would be advantageous to select the speed of rotation of the elevator motor to be as close as possible to the value set by the maximum output voltage of the power supply apparatus. In this case when the speed of rotation is increased, however, a problem forms in that repetitive voltage reductions/electricity outages might cause unnecessarily many breaks in the operation of the elevator.
In some cases it would be advantageous to increase the speed of the elevator car to be even higher than the rated speed in situations in which the net load of the elevator, i.e. the force difference acting in the elevator ropes on opposite sides of the traction sheave, is smaller than the rated load. Also in this case operating breaks of the elevator caused by voltage reductions/electricity outages might form a problem.